1. Field of the Invention
The present invention relates in general to an automotive power steering gear, and more particularly to a hydraulic valve device of an automotive power steering gear.
2. Description of the Prior Art
In order to control feeding of hydraulic fluid to a power cylinder, a hydraulic valve device is connected to a power steering gear.
For clarification of the task of the present invention, one conventional hydraulic valve device will be described with reference to FIGS. 6, 7 and 8 of the accompanying drawings.
FIG. 6 shows a hydraulic circuit of an essential part of the conventional hydraulic valve device.
In the drawing, denoted by numeral 6 is a hydraulic pump, and denoted by numeral 7 is a reservoir tank. Denoted by numeral 10 is a fluid line which extends from the hydraulic pump 6 to a branch point D1 from which two fluid lines 10a and 10b extend. These two fluid lines 10a and 10b are mated at a junction point D2 from which a fluid return line (no numeral) extends to the reservoir tank 7.
The fluid line 10a has two variable apertures 1R and 2L mounted thereto, and the other fluid line 10b has two variable apertures 1L and 2R mounted thereto. These variable apertures 1R, 2R, 1L and 2L are formed in an after-mentioned hydraulic valve 41 installed in the steering gear.
A power cylinder 5 is incorporated with the two fluid lines 10a and 10b, which has one I/O opening connected to an intermediate point between the variable apertures 1R and 2L and the other I/O opening connected to an intermediate point between the variable aperatures 1L and 2R.
With turning of the steering wheel to the right, the variable apertures 1R and 2R decrease their opening depending on the steering torque. While, with turning of the steering wheel to the left, the other variable apertures 1L and 2L decrease their opening depending on the steering torque.
Although not shown in the drawing, a solenoid valve controlled by a control unit is employed to vary the hydraulic pressure in the power cylinder 5 in accordance with a vehicle speed. That is, when the vehicle runs at a low speed or at a standstill, relatively high assisting force is produced by the power cylinder 5 for setting the driver a relatively light steering job, and when the vehicle runs at a high speed, relatively low assisting force is produced by the power cylinder 5 for setting the driver a relatively heavy steering job.
The fluid line 10 has a point E from which a fluid line extends a chattering protection seal 16 to supply the seal 16 with a hydraulic pressure.
With the above-mentioned arrangement, the hydraulic pressure applied to the chattering protection seal 16 depends on the assisting force produced by the power cylinder 5.
FIG. 7 shows a sectional view of a power steering device.
In the drawing, denoted by numeral 41 is a hydraulic valve which comprises a valve body 34, a valve shaft 33, a torsion bar 35, an inlet port 36, an outlet port 37 and a return port 38. It is to be noted that the illustration (viz., the illustration shown at the upper left portion of FIG. 7) shown in the zone enclosed by a phantom line is a laterally sectional view of the hydraulic valve 41. The detail of this hydraulic valve 41 is described in page 29 of NISSAN's SERVICE MANUAL published 1981.
FIG. 8 is an enlarged view of a portion of the hydraulic valve 41 of FIG. 7.
In FIG. 8, the seal 16 for suppressing the chattering is shown. As shown, the seal 16 is arranged between the valve shaft 33 and the valve body 34 and applied with a hydraulic pressure which is transmitted through a conduit 16a.
With provision of this seal 16, undesired vibration of the hydraulic valve 41, which tends to occur at a deep steering, is suppressed.
However, in the above-mentioned hydraulic valve device, the hydraulic pressure applied to the chattering protection seal 16 is forced to vary as the opening of the variable apertures 1R, 1L, 2R and 2L changes, that is, when the steering torque changes.
When the hydraulic pressure to the chattering protection seal 16 changes, the slide torque of the hydraulic valve 41 (that is, the friction between the valve shaft 33 and the valve body 34) is varied. This will be seen from the graph of FIG. 5.
Accordingly, in the above-mentioned valve device, the slide torque of the hydraulic valve 41 is varied in response to turning of the steering wheel even when the vehicle is under a high or middle speed movement. This gives uncomfortable steering feeling to the driver.